The following provides an engineered, actionable selection guide to help you rapidly determine the preferred choice of low/medium/high molecular weight PolyDADMAC based on water quality characteristics, treatment objectives, process location, and operating conditions. It includes trial, dosing, and on-site considerations.
I. Performance Overview of Three Molecular Weight Categories (Quick Reference)
Low molecular weight (<100 kDa)
Rapid dissolution, strong charge neutralisation, suitable for low turbidity or applications requiring rapid response.
Medium molecular weight (100–500 kDa)
Charge neutralisation + moderate bridging capability, versatile ‘general-purpose’ choice.
High molecular weight (>500 kDa)
Significant bridging and floc strength; suitable for high turbidity, sludge dewatering, or applications requiring enhanced solid-liquid separation efficiency; dissolution requires attention.
II. Decision-Making by Project Objective (One-Step Solution)
Objective: Rapid decolourisation / Colloidal charge removal / Drinking water pretreatment
→ Prioritise low molecular weight. Rationale: Rapid reaction, low residue, easy control.
Objective: Conventional municipal/industrial coagulation-sedimentation (balancing cost and efficacy)
→ Prioritise medium molecular weight. Rationale: Capable of both neutralisation and producing moderate flocs.
Objective: High turbidity wastewater, oily/emulsified systems, sludge dewatering/filter pressing
→ Priority: High molecular weight (or high molecular weight + CPAM coordination). Reason: Strong bridging, large stable flocs, excellent dewatering.
Target: Synergistic enhancement with inorganic coagulants (e.g., PAC)
→ Commonly uses medium-low molecular weight PolyDADMAC for electro-neutralisation/enhancement, followed by or concurrently with PAC to optimise sedimentation.
III. Decision-making based on water quality characteristics (more refined)
Low turbidity (SS < 20 mg/L, drinking water grade) → Low molecular weight preferred; minimal dosage to avoid over-dosage.
Medium turbidity (20–200 mg/L) → Medium molecular weight preferred; balances flocculation speed and floc strength.
High turbidity (>200 mg/L) or oily effluent → Prioritise high molecular weight; or sequential PolyDADMAC → CPAM.
High organic load / high colour (COD, substantial humic substances) → Low/medium molecular weight first neutralises organic colloids, then combine with PAC or adsorbents as required.
High salinity/hardness water → Verify salt tolerance and formulation; medium molecular weight or specialised formulations typically offer greater stability.
IV. Testing Protocol (Mandatory: Jar test + small-scale dewatering trial)
Preparation: Obtain representative raw water (≥5 L), prepare samples of ≥3 molecular weight grades (or different brands).
Gradient dosing: Conduct 4–6 concentration gradients per molecular weight (e.g., 0.5, 1, 2, 5 mg/L; higher dosages for dewatering).
Agitation protocol: High-speed agitation for 1–2 minutes (to disperse flocculant), medium-speed for 2–5 minutes (to promote collision/flocculation), followed by settling for 10–30 minutes.
Recording: Effluent turbidity, sedimentation rate, floc size and stability (hand rub test), cake solids content (if dewatering).
Small-scale filter press/centrifuge: If dewatering is the objective, conduct sludge dewatering tests (measure cake solids content and filtrate).
Final evaluation: Balance performance, dosage (mg/L), cost (¥/kg), and operational convenience.
V. Dosage and Process Control Essentials (Field Implementation Guidance)
Dissolution and Preparation: High molecular weight products require slow dissolution under low shear mixing to prevent degradation from excessive shear. Liquid formulations are ready-to-use upon opening or prepared as per manufacturer's recommended working solution.
Dosage Point Selection: Prioritise charge neutralisation (raw water or pre-coagulation stage); dewatering agents (high molecular weight) should be added to the concentrated slurry stage prior to centrifugation/filter pressing.
Mixing Energy: Rapid agitation for dispersion, slow agitation for stable floc formation; high molecular weight products are susceptible to degradation by high shear.
pH and Temperature: Maintain pH within 4–9 range. Note that low temperatures (<10°C) require increased dosage or extended contact time.
Initial Dosage Reference Range (Experimental Starting Point Only):
Low molecular weight: 0.1–5 mg/L (clarification/drinking water)
Medium molecular weight: 1–20 mg/L (conventional coagulation)
High molecular weight: 10–300 mg/L (dewatering/high turbidity/oil-containing, adjust based on sludge characteristics)
Note: Jar test results prevail; significant field variations exist.
VI. Formulation and Cost-Balancing Recommendations
Do not compare unit prices alone (¥/kg); calculate unit treatment costs (¥/m³ = dosage × unit price).
In most operating conditions, medium molecular weight achieves stable results at lower dosages, representing the most cost-effective starting point.
Where minimising sludge volume or maximising solids content is the objective, high molecular weight (or combined with CPAM) often reduces overall disposal costs.
VII. Common Issues and Troubleshooting (Rapid Diagnosis)
Small flocs, slow settling → Possible causes: insufficient molecular weight, inadequate dosage, or unsuitable pH.
Loose, fragile flocs → Possible causes: excessive shear stress (breaking polymer chains) or need to switch to higher molecular weight.
Effluent rebound (re-flocculation) → Possible over-dosage or improper dosing sequence/mixing; conduct zeta potential testing and adjust to near-zero.
High viscosity, poor solubility → Improper dissolution of high molecular weight polymer; switch to slow preparation or select a lower viscosity grade.